Method and apparatus for grinding or honing



Feb 7, R950 L. E. STERNS 2,497,92l

METHOD AND APPARATUS FOR GRINDING OR HONING Filed Aug. 19, 1946 2 Sheets-Sheet 1 INVENTOR. L RANCE 5 511m Feb. 7, 1950 E. STERNS METHOD AND APPARATUS FOR GRINDING OR HONING 2 Sheets-Sheet 2 Filed Aug. 19, 1946 INVENTOR. I Law we: 5 Srmm Patented Feb. 7, 1950 UNITED STATES PATENT OFFICE.

METHOD AND APPARATUS FOR GRINDING OR HONING 14 Claims.

This invention relates to improvements in methods and apparatus for grinding or honing the inner surfaces of hollow members. More particularly, this invention relates to improvements in methods and apparatus for grinding or honing the inner surfaces of hollow members by means of abrasive particles carried in a fluid medium.

It is, therefore, an object of the present invention to provide improvedmeth'od's and apparatus for grinding or honing the inner surfaces of hollow objects by abrasive particles carried in a fluid medium.

In manufacturing hollow articles, it is often desirable to provide an extremely smooth and highly polished surface on the interior of those articles. In some instances, an article can be bored and reamed togive it an interior surface that is smooth enough to meet the specifications set for the article. In other instances, however, surfaces obtained by boring and reaming are not smooth enough to enable the article to meet the specifications set for it; and in such instances, additional treatment must be given the bored and reamed surfaces to remove any minute marks left by the boring and reaming tools. This additional treatment is customarily provided by hones; and the homes can provide highly polished surfaces that are extremely smooth. Unfortunately, mechanical and structural. factors set a limit on the size of the opening or recess into. which a. hone can be fitted; for example, shotgun barrels and the tubes of some hydraulic: systems have such small diametersv that a hone cannot be used. to hone their interior surfaces. However, it is important that these and other articles with small-diameter bores. or recesses be provided with a highly polished interior surface. Presently known. methods and apparatus of treating metal surfaces that might seem to be capable of pro viding the required surface for articles of this type are actually unsuitable and unusable. One of these methods utilizes a rod. or shaft that extends within an object and directs a. stream of air and shot, broken glass or granulated slag against the inner surface of that object. This method is presently being used inthe removal of boiler scale from the interior of boilers. or in the removal of slag from the interior of. furnaces; and in this method the rod or shaft extends into the boiler or furnace and'causes the mixture of air andshot. broken glass or granulated slag to be. forcibly ejected from the end. of the rod or shaft. The mixture is directed against the surface. of the hollow object by means of a nozzle or deflector at the end of the rod. or shaft and will impinge. against the inner surface of the. boiler or furnace with sufficientforce to. dislodge the boiler scale, slag or other material adherent to that surface. In. some instances'the mixture is l 2 projected outwardly from the rod or shaft in a radial direction and strikes the inner surface of the boiler or furnace at right angles to that surface. In other instances, the mixture is projected toward the inner surface of the boiler or furnace in such a way that the stream of air'and particle'sis'inclined at an angle of approximately forty-five degrees relative to that inner surface. In all such instances, the rate of flow will combine with the angle of incidence to provide such an impact between the mixture and the boiler scale, slag or other material that the boiler scale, slag or other material will be dislodged. Care must be used with such methods because the rate of flow and the angle of incidence are so" high that the shot or the glass orthe slag will cut away the metal of the boiler or furnace whenever the boiler scale, slag or other material is dislodged. Moreover, the impact of the shot or glass 'or' slag on the inner surface of the boiler or furnace is so great that the inner surface is marked and scarred. For these reasons, the methods used in removing scale or slag from the inner surfaces of boilers or furnaces cannot be used to provide highly polished smooth surfaces. Instead of improving the bored and reamed surface as required, this method would so scar and pit the surface as to require another application of the reamer.

Another method, that might seem to be capable of additionally smoothing and polishing a bored and reamed surface but actually cannot do so, is the method of abrading scale from. an object. In this method, a mandrel is provided with annular ture of air and abrasive particles against the inner surface of a. hollow object. Once again, however, the rate of. flow and the angle of incidence are such that the mixture strikes the inner surface of the object with sufficient impact to dislodge scale or other material on. that surface. Once again, great care must be taken to halt the operation as. soon as the scale is dislodged because the mixture will remove part of the metal itself and leave a scarred and pitted surface. This removal of metal will be particularly serious since the ribs. on the mandrel will concentrate the mixtureand will localize its effect, thus creating a series of annular recesses. in thesurface of the object. For these reasons this second. method. also would impair rather than improve a bored and reamed surface. Still another method that might seem capable of improving the inner surface of. an object consists inv forcing a. mixture of air and abrasive particles through the object. in contact with and parallel to the inner surface of the hollow object. Such .a method cannot provide auniformly smoothinn'er surface for the object because imperfectionsin the. surface of the object. willt'end. to stratify, the

ribs that haveinclined faces which direct amixfluid-medium-held particles that is positively directed along the inner surface of bored and reamed objects. More particularly, the present invention directs a helical stream of fluidmedium-held abrasive particles along the inner surface of an object while it causes relative rotation between said stream and said surface. It is, therefore, an object of the present invention to mix abrasive particles with a fluid medium, and to direct a helical stream of said mixture along the inner surface of an object while causing relative rotation between said stream and said inner surface.

The present invention does this by providing a rotatable spindle which has fluid-directing means that cause the fluid-held abrasive parduced from the passage 14 by the worm section I! of shaft I 6 are permitted to mix with air which is introduced into the chamber I8 through the passage 20. The passage 20 extends upwardly from the chamber l8 and it has a pressureequalizing duct 22 extending from one side thereof into chamber l0, and it has a fluid inlet 24 f openin into the other side thereof. Positioned V 'rotatably within the passage 20 and the chamber ticles to follow a helical path and to flow past each and every section of the interior of the hollow object. These fluid-directing means provide positive control over the fluid-held particles and they prevent turbulence or stratification of the medium; and the relative rotation between the spindle and the inner surface of the object provides uniform grinding or honing of that entire inner surface. It is, therefore, an object of the present invention to provide a rotatable mandrel which has fluid-directing means that cooperate with a mixture of abrasive particles and a fluid medium to provide a smooth and uniform grinding or honin of the inner surfaces of hollow objects.

Other objects and advantages of the present invention should become apparent from an examination of the drawing and accompanying description.

In the drawing and accompanying description, two preferred embodiments of the present invention are shown and described but it is to be understood that the drawing and accompanying description are for the purpose of illustration only and do not limit the invention and that the invention will be defined by the appended claims. In the drawing- Fig. 1 is a cross sectional view of a grinding or honing machine embodying the principles and teachings of the present invention, and

Fig. 2 is a partial cross sectional view of a second type of grinding or honing machine made in accordance with the present invention.

Referring to the drawing in detail, the numeral [0 denotes a chamber which is hollow and has a generally conical bottom. This chamber has a removable top I2 through which minute abrasive particles may be introduced into the chamber I 0. Communicating with the conical bottom of the chamber i0 is an abrasive-feeding passage I4 which encloses a rotatable shaft [6 that has a worm section I! formed thereon. The Worm section II on the shaft l6 extends from the opening at the bottom of the chamber H! to the side wall of the chamber l8. Chamber i8 is contiguous with the passage I4, and it is a mixing chamber wherein the abrasive particles intro- [8 is a shaft 26 which has a helical groove 28 ,formed in the lower portion thereof. The shaft '26 will be of a wear-resistant material such as hardened steel or synthetic rubber, it will be sealed to the upper end of the passage 20 by a sealing means, not shown, of the conventional type, and it will be freely rotatable relative to the passage 20 and the chamber [8. The shaft 26 is also provided with a circular flange 39 which is located adjacent the bottom of the passage 20 and which cooperates with passage 20 to form an annular opening of small proportions between the passage 20 and the chamber l8. This small annular opening cooperates with the fluid introduced through inlet opening 24 to provide a continuous jet of fluid which will, upon entering chamber l8, stir up and pick up the particles of abrasive introduced into chamber l8 from the passage l4.

The bottom of chamber I8 is of generally conical shape and it has a relatively large opening therein. Positioned within this opening is a generally annular sealing member 32 which has an annular seating shoulder 34 thereon that is engageable with an annular recess adjacent the opening at the base of the chamber l8. The sealing member 32 has flexible ribs 36 which can easily be bent upwardly to permit insertion of the work piece 38, shown by dotted lines, into the base of the chamber I 8, and which will then by reason of their own resilience and by reason of the fluid pressure in the chamber l8 be held tightly against the work piece 38. In this way, the sealing member 32 permits ready insertion of the work piece 38 without permitting subsequent leakage of fluid or abrasive along the outer surface of the work piece 38. The work piece 38 extends coaxially with the shaft 26 and it encircles the grooved section 28 of that shaft for a major portion of its length. The lower end of the work piece 38 is supported by an annular recess 40 in the combined exhaust duct and work support 42. The support 42 is so dimensioned that the recess 40 therein closely engages the bottom of the work piece 38 and prevents loss of air or abrasive material there between. The exhaust duct and work support 42 is preferably connected to a separating device which will separate the abrasive from the fluid medium preparatory to the reuse or discharge of the fluid medium or the reuse or discharge of the abrasive particles.

In assembling the individual elements of the apparatus of Fig. 1, the housing which contains the chamber Hi, the passage 20, the chamber l0 and the passage [4 could be made so it could divide into two sections along the plane indicated by the cross-hatching of Fig. 1 or a parallel plane; and after the two sections of that housing had been separated the shaft I 6 with its worm section I! could be inserted in the abrasive-feeding passage l4. Thereafter, the resilient annulus 32 could be slipped over the helically formed end of rotatable shaft 26, and then the haft and the annulus could be placed in the position shown in Fig. 1. Thereupon, the two sections of the housing could be brought together and firmI-ysecuredln fluidtight relation.

' In operating the apparatus of Fig. 1, the removable top l2 of chamber can be removed and abrasive particles introduced into' chamber to. Thereafter, upon the replacement of top 12 and the securement of shafts I56. and 26 to motors or other rotation-imparting devices and the connection. ofv inlet 24 to a source of high pressure fluid, the apparatus is ready to receive the work piece 3'8. To insert the work piece, the combinedduct and support 42 is moved downwardly away from the lower end of shaft 26, the work piece 38 is telescoped over the end of the shaft 26 and is moved upwardly until its upper end engages and slips past" the resilient ribs 36 of. the annular sealing member 32 to the position shownin Fig. 1. Thereafter, the combined exhaust duct and work support member 42 can be moved upwardly until it encloses the lower end of the shaft 26 and receives the bottom of the workpiece 38. If desired, the work piece 38' can be initially telescoped upwardly into chamber [8 beyond the position shown in Fig. 1, the combinedwork support and exhaust duct 42 can be raised into the position shown in Fig. 1, and then the work piece 38 can be moveddown from its telescoped position inside of chamber l8 until the bottom of the work piece 38 rests in and presses against the recess 40 of. the combined exhaust duct and work support 42. Also, if desired, the passage 2!? can be made considerably longer than Fig. 1 shows it to be. In that case, the shaft 25 canbe retracted entirely within the passage 25 and the work piece can be set in position and can be removed without necessitating any movement of the exhaust duct and work support 62. Once the shaft 26 has been retracted into passage 20, the work piece 38 can be set at a slight angle to the position shown in Fig. 1 and can have its upper end telescoped'past the re"- silient ribs 3%- into the chamber [8 until its lower end clears'the upper end of the combined exhaust duct and work support 42'. Thereafter the work piece 38 can be rotated into the position shown in Fig. 1 and can be moved downwardly until its lower end engages and is held by the recess 40. Thereuponthe rotatable shaft can be moved to the position shown in Fig. 1. Removal of the work piece 38 can also-be effected without movement of the combined exhaust duct and work support 42 by performing the above operations in reverse order.

Once the work piece 38 has been placed inthe position shown in Fig. 1, a. high pressure fluid medium such as air, carbon dioxide, steam or other suitable gas or vapor is introduced into the chamber is by way of inlet 24 and passage and this medium will admix with the abrasive particles introduced into chamber l8 from passage 14. The resulting mixture of fluid-held particles will pass between the inner surface of the work piece 38 and the outer surface of the. shaft 26; and in doing so the mixture will be caused to follow a helical path.- In following that path the mixture will be held against turbulence or stratification even if small obstacles, tool marks or other projections extend above the rest of the inner surface of the work. piece 38. In addition the mixture will, because of centrifugal force and because of its inertia, move along in contactwith the inner surface of the work piece 38 and act to grind or hone that surface; Such grinding or honing will be quite uniform over the entire inner surface of the work piece 6*). 38 since the rotation of the 'shaft- 26 will bring the helical stream of abrasive and fluid medium into contact with every'partof the" inner surface of the workpiece 38.

The shaft 26- will preferably be only a few thousandths of an inchsmall'er in diameter than the innerdiameter of; the work piece 38, although the-drawing has been madeto show an annular: section ofv considerable thickness between the inner diameterofvworkpiece 38 and the outer diameter of the shaft 26. Such a showing has been made to facilitate full and clear illustration of thenpresent invention. The limited clearance betweenthe inner diameter of the work piece 38 and theouter diameter of the shaft 26 is very desirable because-it will provide sufiicient space for the mixture of abrasive and fluid medium to pass but will permit the helical groove 28 to positively direct the mixture along a helical path. Moreover by having the space between the shaft 25 and the inner surface of the work piece 38 of such small area, the present invention makes it possible for the mixture to assist in holding the shaftZt in the axial. center of the work piece 38.

The structureof Fig. 2 differs to some extent from the structure of Fig. 1, since the structure of Fig. 2 is directed to grinding or honing the inner surfaces of hollow objects which have closed ends. In Fig. 2, the numeral M denotes a chamber which receives abrasive particles and which'has a removable cap 46. The chamber 4'4 has a generally conical bottom with an opening therein, and this opening communicates with abrasive-feeding passage '48 in which is positioned' a shaft 50. Shafttil', like shaft It of Fig. 1, has a worm section '5! on one end thereof. Feeding passage 48 communicates with mixing chamber 52, which chamber is also connected to the passage 54. At the top of passage 54 are a pressure-equalizing passage 56 which extends over to the abrasive chamber 44, and a duct 58 which can be connected to a source of high pressure fluid. Positioned within chamber 52 and passage 5.4 is rotatable shaft 60. Unlike the shaft 26 of Fig. 1, shaftBfl' is hollow; and the hollow center of shaft 60 extends upwardly beyond the top of passage 54 and communicates with an exhaust duct 68. The upperend of shaft 60 is provided with a shaft-sealing means 62', a pulley 6'4, and a second shaft-sealingm'eans. 66. The shaftsealing means 66 is positioned within the exhaust duct 68; and the exhaust duct'GB is preferably connected to a. separating device that will receive the mixture of. abrasive and fluid medium and will cause the abrasive to be separated from the fluid medium so the abrasive or the fluid medium orboth'canbe' reused. Connected to the lower end of'the shaft 60', as by a press fit, is a sleeve 12 which is provided with a helical groove 14 on theexterior thereof. This sleeve is dimensioned to fit within the work piece It, which is shown in Fig. 2 by dotted lines; and this work piece is dimensioned to fit within the annulus N3 of resilient material which is secured tothe generally conical bottom-of the chamber In operatingthis device, the work piece 16 is forced upwardly intoengagement with the resilient work holding means 10, abrasive is introduced into the chamber 44 through the opening from which top 46 has beenremoved, the top 46 is replaced, a source of high pressure fluid medium is connected to the duct 58, and pulley 64- is bolted to a source of power and is caused to rotate. Thereafter, the fluid medium will pass from the chamber 54 into the chamber 52 where it will admix with abrasive particles forced through the feeding passage 48 by the worm section of the shaft 50. The resulting mixture will pass downwardly between the exterior of the sleeve 12 and the interior of the work piece '16 until it has passed below the bottom of the sleeve 12, and thereafter it will pass inwardly of the shaft 60 and start upwardly until it passes into and through the duct 68. As the mixture passes along the length of the sleeve 12, it will be caused to follow a generally helical path by reason of the helical groove 14 in the sleeve 12 and by rea son of the small gap between the interior of the work piece '16 and the exterior of the sleeve 12. Once again, the drawing has, to facilitate clearer illustration of the present invention, shown a larger gap between sleeve 12 and the interior of the work piece 16. The groove 14 in sleeve 12 will act in the same manner as the groove 28 of shaft 26 acted in Fig. 1, and it will provide a directed path of helical form for the mixture. The rotation of the shaft 60, caused b belting the pulley 64 to a source of power, will provide a uniform movement of the helical stream of mixture relative to the inner surface of the work piece 16. By dimensioning the size of the sleeve 12, which can be removed and replaced by sleeves of different sizes and with grooves of different sizes, it is possible to obtain just exactly the right kind of grinding or honing action on the interior of the work piece 16. Here again, the mixture will be closely confined to predetermined paths; and that mixture will, because of centrifugal force and its own inertia, pass along in contact with the inner surface of the work piece 16.

Because of the centrifugal force and the inertia of the abrasive particles in the mixture of fluid medium and abrasive particles, very little abrading or grinding of the shaft 26 or the sleeve 72 occurs. Most of the abrasive particles will be adjacent to and will be acting upon the inner surface of the work pieces 38 and 16 rather than shaft 26 or sleeve 12.

The amount and fineness of the grinding or hOlllIlg action is determined by the size and nature of the abrasive used, by the pressure and nature of the fluid medium used, the space between the mandrel and the work piece, the depth and lead of the groove, and the rate of rotation of the mandrel. If several different grades of grinding or honing are desired, abrasives of various grades and sizes can be selectively passed over the surface of the work piece. The mandrel 26 has a certain diameter and the groove 28 has a certain configuration, depth, and lead, but the diameter of the mandrel and the depth, configuration and lead of the groove can be varied to meet the requirements of the particular work piece.

The methods and apparatus of the present in vention not only make it possible to hone bores of small diameter, but they also make it possible to provide finished surfaces that are of a higher quality than the surfaces which can be obtained even on large bores with other presently known methods and apparatus. This is due to the fact that the abrasive particles in the present invention slide over the surface while they act to cut or wear away projections or obstructions extending upwardly from the surface, and thus there is no gouging or scratching of the surface itself.

In lapping and honing operations now being used, a portion of the abrasive is loaded into the surface because the abrasive is held against the surface by mechanically imposed pressures and those pressures will cause some of the abrasive particles to be rolled or pressed directly into the surface. This condition is not desirable and it is eliminated b the methods and apparatus of the present invention since it is impossible for the pressures on the mixture to apply so much pressure on the individual abrasive particles that they will embed themselves in the surface. In addition, the closely controlled and regulated cutting action, and the speed at which the method of the present invention can be carried out, makes it economically feasible to apply more than one grain size of abrasive to the Work. All of this enables the present invention to provide finer and more desirable surfaces than can be obtained by present honing and lapping methods. With the use of very fine abrasive flours, still finer surfaces can be produced by the present invention.

Whereas two preferred embodiments of the present invention are shown and described in the drawing and accompanying description, it should be obvious to those skilled in the art that various changes can be made in the form of the invention without changing the scope thereof.

What I claim is:

1. The method of grinding or honing the interior surface of an object that comprises mixing an abrasive with a fluid medium to make an abrasive-laden fluid medium, introducing said abrasive-laden fluid medium into engagement with said object, and thereafter confining said abrasive-laden fluid medium for movement along a helical path in continuous engagement with the surface of said object while causing relative transverse movement of said object and said path, whereby the abrasive in said abrasiveladen fluid medium is directed along the surface of said object and provides smooth grinding or honing thereof.

2. The method of grinding or honing the interior surface of an object that comprises mixing abrasive with a fluid medium to form an abrasive-laden fluid medium, directing said abrasive-laden fluid medium into engagement with said object, and thereafter confining said abrasive-laden fluid medium for movement in a helical path along and in continuous contact with the interior surface of said object, said helii cal path having the axis thereof coaxial with the axis of said object.

3. The method of grinding or honing the interior surface of an object that comprises forming an abrasive-laden fluid medium, directing said abrasive-laden fluid medium toward said object and introducing said abrasive-laden fluid medium into the interior of said object in a direction generally parallel to the interior surface of said object, confining said abrasive-laden fluid medium for movement in a helical path along the interior surface of said object, and causing movement of said path relative to said interior surface to obtain uniform grinding or honing of said interior surface.

4. The method of grinding or honing the interior surface of an object with a cylindrical interior surface that comprises forming an abrasive-laden fluid medium, directing said abrasive-laden fluid medium into engagement with the cylindrical interior surface of said object in a direction generally parallel to the axis of the interior surface of said object, confining said abrasive-laden fluid medium in a helical path along the interior surface of said object, and moving said path circumferentially of the interior surface of said object to bring said abrasive-laden fluid medium into contact with all portions of said interior surface.

5. The method of grinding or honing the interior surface of an object that comprises admixing abrasive with a fluid medium, introducing said mixture into the interior of said object in a stream, and confining said stream for movement in contact with the interior surface of said object along a curved path.

6. Grinding or honing apparatus that comprises a mixing chamber, means to introduce abrasive into said chamber, a fluid-medium inlet for said mixing chamber, an outlet from said mixing chamber, a work holder adjacent said outlet, and a helically grooved mandrel that extends into said mixing chamber and extends into work held by said work holder.

7. Grinding or honing apparatus that comprises a fluid-directing mandrel, a support for said mandrel, a work support, means to introduce a mixture of abrasive particles and a fluid medium between said mandrel and a work piece held by said work support, and means to cause relative rotation between said mandrel and said work piece.

8. Grinding or honing apparatus that comprises an abrasive storage chamber, a mixing chamber, means to move abrasive from said storage chamber to said mixing chamber, means to introduce fluid medium under pressure into said mixing chamber, a conduit between said mixing chamber and said storage chamber to equalize the pressures in said chambers, an outlet from said mixing chamber, a work support adjacent said outlet, and a helically grooved mandrel that extends into said mixing chamber and into a work piece held by said work support, and means to provide relative rotation between said mandrel and said work piece.

9. Grinding or honing apparatus that comprises a mixing chamber, means to introduce abrasive into said mixing chamber, a fluid-medium inlet for said mixing chamber, an outlet from said mixing chamber, a work holder adjacent said outlet, and a mandrel that extends into said mixing chamber and into work held by said work holder, said mandrel having fluiddirecting means therein to direct a mixture of fluid-held abrasive particles through the interior of said work in a path inclined to the axis of said work.

10. In a grinding or honing apparatus that comprises a mixing chamber which has a fluidmedium inlet, an inlet for abrasive, an outlet for abrasive-laden fluid medium, and means to direct abrasive-laden fluid medium across the surface of a work piece adjacent said outlet, the improvement of a work holder adjacent said outlet that has resilient ribs inclined toward said mixing chamber, said ribs permitting ready insertion of a work piece into said work holder but acting to maintain said work piece in position adjacent said outlet and to prevent leakage of said abrasive-laden fluid medium between said work holder and said work piece.

11. Grinding or honing apparatus that comprises a support for a work piece, a fluid-directing element adapted to be disposed at least partially within the interior of a work piece mounted on said support, said fluid-directing element being so dimensioned relative to said work piece as to permit the introduction of a shallow stream of abrasive-laden fluid medium into the space between the exterior of said fluid-directing element and the interior ofsaid object, and an enclosure adjacent one end of said work piece to direct an abrasive-laden fluid medium into said space between the exterior of said fluid-directing element and the interior of said work piece, said fluid-directing element confining said abrasive-laden fluid medium in contact with the interior of said work piece while directing said abrasive-laden fluid medium along paths that extend both longitudinally and transversely of the interior of said work piece.

12. In a grinding or honing apparatus of the type that uses an abrasive-laden fluid medium to grind or hone the interior surface of an object, the improvement which comprises a rotatable fluid-directing mandrel disposed at least partially within the interior of said object, bearings that support said mandrel in position to extend at least partially within the interior of said object, a motion-imparting element adapted to rotate said rotatable fluid-directing mandrel, and fluid-directing surfaces on said mandrel to positively direct an abrasive-laden fluid medium along paths which are in contact with the interior surface of said object but are inclined to the axis of said object, the rotation of said mandrel moving said paths transversely of the interior surface of said object.

13. The method of grinding or honing a surface that comprises mixing particles with a fluid medium, positioning a fluid-directing member adjacent to but spaced from a work piece, and directing the mixture of fluid medium and abrasive particles between said fluid-directing member and said work piece in a direction generally parallel to the axis of said fluid-directing member while causing said mixture to flow transversely and longitudinally along the surface of the work piece in curved paths and thereby remove tool marks or other obstructions projecting up from said surface.

14. The method of grinding or honing a surface that comprises mixing a fluid medium with abrasive particles and then moving the fluidheld particles across, rather than directing those particles angularly against, the surface to be ground or honed While confining the fluid-held particles in positively directed paths of shallow depth, said paths being directed both transversely and longitudinally of said surface.

LORANCE E. STERNS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

